Bring a large pot of salted water to a boil and cook pasta according to package instructions until al dente. Drain and rinse under cold water to cool completely.
In a large skillet over medium heat, melt butter and olive oil together. Add minced garlic and cook for 1-2 minutes until fragrant, being careful not to burn.
Add shrimp to the skillet and sauté for about 2-3 minutes until just cooked through. Add crab meat and lemon juice, and gently toss to coat. Remove from heat and season with salt and pepper to taste.
In a large mixing bowl, combine cooled pasta, the shrimp and crab mixture, red onion, and chopped parsley. Add garlic butter mixture and toss gently to coat everything evenly.
Chill in the refrigerator for at least 30 minutes before serving to allow flavors to meld.
Optionally garnish with chopped basil and lemon slices before serving. Serve chilled.